As is well known, natural gas defines a very broad range of gas compositions. Methane is the largest component of produced natural gas, and usually accounts for at least 80% by volume of what is known as marketable natural gas. Other components include, in declining volume percentages, ethane (3%-10%), propane (0.5%-3%), butane and C4 isomers (0.3%-2%), pentane and C5 isomers (0.2%-1%), and hexane+and all C6+ isomers (less than 1%). Nitrogen and carbon dioxide are also commonly found in natural gas, in ranges of 0.1% to 10%.
Some gas fields have carbon dioxide contents of up to 30%. Common isomers found in natural gas are iso-butane and iso-pentane. Unsaturated hydrocarbons such as ethylene and propylene are not found in natural gas. Other contaminants include water and sulphur compounds, but these must typically be controlled to very low levels prior to sale of the marketable natural gas, regardless of the transport system used to get the produced gas from wellhead to market.
Secord and Clarke in U.S. Pat. Nos. 3,232,725 (1963) and 3,298,805 (1965) describe the benefits of storage of gas at conditions of temperature and pressure which occur when the gas exists at a single dense phase fluid state, at pressures just above the phase transition pressure. This state is shown in the generic phase diagram (taken from U.S. Pat. No. 3,232,725) attached hereto at FIG. 12, and is shown as occurring within the dotted lines on the diagram.
The relation between pressure, volume and temperature of a gas can be expressed by the Ideal Gas Law, which is stated as PV=nRT where, using English units:
P=pressure of the gas in pounds per square inch absolute (psia)
V=volume of the gas in cubic feet (CF)
n=number of moles of the gas
R=the universal gas constant
T=temperature of the gas in degrees Rankin (degrees Fahrenheit plus 460)
The Ideal Gas Equation must be modified when dealing with hydrocarbon gases under pressure, because of the intermolecular forces and the molecular shape. To correct for this, an added term, the compressibility factor z must be added to the Ideal Gas Equation such that PV=znRT. This z is a dimensionless factor that reflects the compressibility of the particular gas being measured, at the particular conditions of temperature and pressure.
At or near atmospheric pressure, the z factor is sufficiently close to 1.0 that it can be ignored for most gases, and the Ideal Gas Equation can be used without the added z term.
However, where pressures exceed a few hundred psia the z term can be much lower than 1.0 so that it must be included in order for the Ideal Gas Equation to give correct results.
According to the van der Waal's theorem, the deviation of a natural gas from the Ideal Gas Law depends on how far the gas is from its critical temperature and critical pressure. Thus, the terms Tr and Pr (known as reduced temperature and reduced pressure respectively) have been defined, whereTr=T/TcPr=P/PcWhere,
T=the temperature of the gas in degrees R
Tc=the critical temperature of the gas in degrees R
P=the pressure of the gas in psia
PC=the critical pressure of the gas in psia
Critical pressures and critical temperatures for pure gases have been calculated, and are available in most handbooks. Where a mixture of gases of known composition is available, a “pseudo critical temperature” and “pseudo critical pressure” which apply to the mixture can be obtained by using the averages of the critical temperatures and critical pressures of the pure gases in the mixture, weighted according to the mole percentage of each pure gas present. The pseudo reduced temperature and the pseudo reduced pressure can then be calculated using the pseudo critical temperature and the pseudo-critical pressure respectively.
Once a pseudo reduced temperature and pseudo reduced pressure are known, the z factor can be found by using standard charts. An example of one of these is “FIG. 23-3 Compressibility Factors for Natural Gas”, by M. B. Stranding and D. L. Katz (1942), published in the Engineering Data Book, Gas Processors Suppliers Association, edition (Tulsa, Okla., U.S.A.) 1987 (and a copy of that chart is attached hereto as FIG. 13).
One aspect of the prior art is described in U.S. Pat. No. 6,217,626 “High pressure storage and transport of natural gas containing added C2 or C3, or ammonia, hydrogen fluoride or carbon monoxide”. That patent describes a method for storing and subsequently transporting gas by pipeline whereby adding the light hydrocarbons of ethane and propane (or ammonia, hydrogen fluoride or carbon monoxide) can increase the capacity of the pipeline or can reduce the horsepower required on a pipeline to propel such a gas mixture down the line. The primary claim is for creating a mixture by addition of propane of ethane where the product of the z factor (z) and the molecular weight (MW) for the new mixture reduces as compared to a mixture without the added ethane or propane, yet where there is no presence of liquids, only a single phase gas vapor.
The benefit arises because of the gas pipeline flow equation. There are several forms of this equation, but they all have the following features in common:Flow=constant 1 [((P1^2−P2^2)/(S*L*T*z))^0.5 ]*(D^2.5)Where:
PI=starting pressure in a pipeline
P2=ending pressure in a pipeline
S=specific gravity of the gas (which is equivalent to molecular weight)
L=length of the pipeline
T=temperature of the gas
z=compressibility factor of the gas
D=internal diameter of the pipeline
In this equation, the two factors that are altered by changing the gas composition are the specific gravity (or molecular weight) “S”, and the z factor “z”. Both of these appear in the denominator of the equation. Therefore, if the product of z and MW or “S” reduces, and all other factors remain constant, flow on the pipeline will increase at a similar pressure differential between the starting and ending points. This is a benefit in pipeline transmission, which can be described either as a capacity gain or a reduced horsepower requirement to propel a given volume down the pipeline.
The primary claim in the U.S. Pat. No. 6,217,626 is adding C2 or C3 to natural gas for a reduction in the product of z and MW (or S), above a pressure of 1000 psig and with no discernible liquid formation. The benefits described under the patent relate to increased capacity or reduced horsepower on a pipeline.
The teachings under the patent describes a mixture in which the primary barrier to increasing benefits is the two-phase state created if too much NGL is added to the gas. This two-phase state leads to physical damage of the pipeline equipment, and reduced flow, and must be avoided. Several of the subsequent claims limit the amount of ethane to 35% and the amount of propane to 12% in order to avoid this two-phase state on the pipeline. Several of the claims state a minimum amount of added ethane and propane, again based on the benefits in pipeline application. No mention is made in U.S. Pat. No. 6,217,626 of adding any hydrocarbons heavier than propane, such as butane or pentane, and in fact, the teachings describe how these heavier hydrocarbons should be avoided, as they lead to premature development of the two-phase state. See page 6, “Thus C4 hydrocarbons are not additives contemplated by this invention.” Furthermore, “The presence of more than 1% C4 hydrocarbons in the mixture is not preferred, however, as C4 hydrocarbons tend to liquefy easily at pressures between 1000 psia and 2200 psia and more than 1% C4 hydrocarbons give rise to increased danger that a liquid phase will separate out. C4 hydrocarbons also have an unfavorable effect on the mixture's z factor at pressures under 900 psia so care should be taken that, during transport through a pipeline, mixtures according to the invention that contain C4 hydrocarbons are not allowed to decompress to less than 900 psia and preferably not to less than 1000 psia.
The control mechanism proposed in the '626 invention to avoid the two-phase state is thus the type and amount of NGL added to the mixture. This is because, in a pipeline, temperature and pressure are usually exogenous variables, not subject to any fine degree of control. Refrigeration is mentioned only once in '626, and in a negative sense. While some of the claims deal with mixtures down to a temperature of −40 degrees F., the following statement appears on page 10 of the '626 patent: “Even more preferred pressures are 1350-1750 psia (which gives good results without requiring vessels to withstand higher pressures) and particularly preferred temperatures are 35 to 120 degrees F. (Which do not require undue refrigeration)”. The benefits of the invention are illustrated in the graphs attached to '626, which all terminate at a lower temperature limit of 30 to 35 degrees F. Even though the pipeline flow equation illustrates that pipelines are more efficient at colder temperatures (see the factor T in the denominator), no analysis is provided at lower temperatures. This is primarily because refrigeration is not practical in pipeline applications, as the pipe temperature should be above the freezing point of water, in order to prevent frost build up on and around the pipeline.
It is clear that the invention in U.S. Pat. No. 6,217,626 is based on preparation in storage of a fluid with the stated desire of subsequent pipeline transport, and that no refrigeration is contemplated, that the type and minimum amount of NGL added is limited by the benefits provided in pipeline transport, that the type and maximum amount of NGL added is limited by the two-phase problem which will occur on the contemplated pipeline transmission, and that the pressure regime is limited by the subsequent pipeline transmission. While the prior art implies benefits for both storage and pipeline transport, the storage aspect of the prior art is limited to or by pipeline applications, and does not contemplate storage in containers which are themselves later transported.
Another aspect of the prior art is contained within U.S. Pat. No. 5,315,054 “Liquid Fuel Solutions of Methane and Light Hydrocarbons”. This patent deals with a method to store a liquid product where Liquified Natural Gas (LNG) is put into an insulated tank at a temperature of about −265 degrees F. Both methane and NGL are introduced into the tank, the methane and LNG is dissolved in the NGL hydrocarbon solution (typically propane or butane), and the resulting mixture is stored as a stable liquid under moderate pressure. This invention does not contemplate storage as a single dense phase fluid, and it is also conditional upon LNG being present in the tank to begin with.
Another aspect of the prior art is described in U.S. Pat. Nos. 5,900,515 and 6,111,154 “High energy density storage of methane in light hydrocarbon solutions”. This invention is similar to the previous example 315,054 and is described as the “dissolution of gaseous methane into at least one light hydrocarbon into a storage tank” and “storage of the solution”. In addition, the solution has to be maintained at a temperature above −1 degree C. at a pressure above 8.0 Mpa comprise a maximum of 80% methane and have an energy density of at least 11,000.
Another aspect of the prior art is described in the previously referenced U.S. Pat. No. 3,298,805 which describes storage of natural gas under pressure, without any additives, at or near the phase transition pressure but at a temperature below the critical temperature of methane (−116.7 degrees F.). This is a continuation of U.S. Pat. No. 3,232,725 which describes storing natural gas under pressure, again without any additives, at or near the phase transition pressure at a temperature 20 degrees (F.) below ambient temperature.
Another aspect of the prior arts is described in U.S. Pat. No. 4,010,622 which describes adding hydrocarbons in the range of C5-C20 sufficient to liquefy the gas at ambient pressure and store it as a liquid, which is given as an example with bearing on the formulae expressed above, but not of much relevance to this invention.